Ultrastructural, transcriptional, and functional differences between human reticulated and non-reticulated platelets

Immature platelets and platelet reactivity in patients with acute ST-segment Elevation Myocardial Infarction using whole blood flow cytometry with SYTO-13 staining

BACKGROUND

Reduced effect of antiplatelet therapy has been reported in patients with ST-segment elevation myocardial infarction (STEMI). Multiple factors may concur to explain this, including increased amount of highly reactive immature platelets.

OBJECTIVES

To investigate the association between immature platelets and reactivity determined with multicolour flow cytometry using the SYTO-13 dye in STEMI patients.

METHODS

We conducted an observational study of 59 patients with acute STEMI. Blood samples were obtained within 24 h after admission and after loading doses of dual antiplatelet therapy. For comparison, samples were obtained from 50 healthy individuals. Immature platelets and platelet reactivity were investigated using multicolour flow cytometry including the SYTO-13 dye that binds to platelet RNA and thus provides a method for subdividing platelets into immature and mature platelets. Additionally, we assessed platelet aggregation, serum-thromboxane B2 levels and standard immature platelet markers.

RESULTS

Immature platelets were more reactive than mature platelets in both STEMI patients and healthy individuals (p-values < 0.05). STEMI patients had lower platelet aggregation and thromboxane B2 levels than healthy individuals. We found a positive association between automatically determined immature platelet markers and CD63 expression on activated platelets (Spearman's rho: 0.27 to 0.58, p-values < 0.05).

CONCLUSIONS

Our study shows that immature platelets identified with a multicolour flow cytometric method using the SYTO-13 dye are more reactive than mature platelets in patients with acute STEMI and in healthy individuals. The presence of immature platelets may be important for the overall platelet reactivity, which may have implications for the effect of antiplatelet therapy.

Reticulated Platelets Predict Cardiovascular Death and Adverse Events in Coronary Artery Disease: A Systematic Review and Meta-analysis

BACKGROUND

 The pro-thrombotic immature or reticulated platelets (RPs) are known to be elevated in high-risk patients and in different pathological settings. It has been shown that RPs correlate with an insufficient antiplatelet response to antiplatelet agents. RPs are emerging novel predictors of adverse cardiovascular events in cardiovascular disease. This study, using the totality of existing evidence, evaluated the prognostic role of RPs in patients with coronary artery disease.

METHODS

 We performed a systematic review and meta-analysis including trials of acute and chronic coronary syndrome reporting clinical outcomes according to RPs levels in the peripheral blood. We compared patients with elevated RPs (RPshigh) to patients without elevated RPs (RPslow). Odds ratios (ORs) and 95% CIs were used as metric of choice for treatment effects with random-effects models. The primary endpoint was major adverse cardiovascular and cerebrovascular events (MACCE). Secondary endpoints were cardiovascular death, myocardial infarction, ischemic stroke, urgent coronary revascularization and bleedings.

RESULTS

 A total of 7 studies, including 2213 patients, were included. The risk for MACCE was significantly higher in RPshigh compared to RPslow patients (OR 2.67 [1.87; 3.81], I2  = 43.8%). RPshigh were associated with cardiovascular death (OR 2.09 [1.36; 3.22], I2  = 40.4%). No associations for RPshigh were detected with the other singular components of MACCE: myocardial infarction (OR 1.73 [0.89; 3.38] I2  = 60.5%) and stroke (OR 1.72 [0.59; 4.96] I2  = 21%). The risk of bleeding did not differ between groups(OR 0.58 [0.15; 2.22] I2  = 86.1%).

CONCLUSION

 Elevated RPs are significantly associated with increased risk of cardiovascular events and cardiovascular death.

Prognostic Implications of Immature Platelet Fraction at 5-Year Follow-up Among ACS Patients Treated With Dual Antiplatelet Therapy

Objective: Platelets are strongly associated with cardiovascular events due to their role in thrombotic processes. Reticulated platelets have higher prothrombotic potential. The aim of the study was to evaluate the effectiveness of immature platelet fraction (IPF) in predicting long-term clinical outcomes in patients with acute coronary syndrome (ACS). Methods: This prospective, observational study enrolled patients with ACS treated with dual antiplatelet therapy comprising acetylsalicylic acid and clopidogrel or ticagrelor. The primary outcome was a composite endpoint defined as major adverse cardiovascular events (MACE): all-cause death, myocardial infarction (MI), ischemic stroke, or unplanned revascularization. IPF was determined using flow cytometry in the first 24 h of hospitalization. MACE were evaluated by 2 physicians based on electronic databases and source documentation including discharge letters received from patients upon telephone contact. Results: Overall, there were 140 ACS patients (mean age 65.1 ± 11.7, 37 females [26.4%]) included in this study. Of them, 22.9% had diabetes mellitus, 69.3% hyperlipidemia, 25% had a history of MI. The median IPF values were 2.85 [1.8-4.2] %. Clinical follow-up (median time: 57 months [interquartile range 55-59 months]) was available for 130 patients (92.9%). MACE occurred in 27 patients (20.8%). There were higher rates of MACE at higher IPF tertiles (3rd vs 1st tertile: HR = 5.341 95% CI: 1.546-18.454, P = .008). Cox regression analyses showed that IPF level was independently associated with MACE. Time-dependent receiver-operating characteristic curve analysis revealed area under the curve of 0.656 for 5-year outcome with an IPF cutoff point of 3.45% being 63.0% sensitive and 65.0% specific for MACE. Conclusions: The study showed IPF may be an independent predictor of long-term mortality and MACE (ClinicalTrials.gov number, NCT06177587).

Phosphatidylthreonine is a procoagulant lipid detected in human blood and elevated in coronary artery disease

Aminophospholipids (aPL) such as phosphatidylserine are essential for supporting the activity of coagulation factors, circulating platelets, and blood cells. Phosphatidylthreonine (PT) is an aminophospholipid previously reported in eukaryotic parasites and animal cell cultures, but not yet in human tissues. Here, we evaluated whether PT is present in blood cells and characterized its ability to support coagulation. Several PT molecular species were detected in human blood, washed platelets, extracellular vesicles, and isolated leukocytes from healthy volunteers using liquid chromatography-tandem mass spectrometry. The ability of PT to support coagulation was demonstrated in vitro using biochemical and biophysical assays. In liposomes, PT supported prothrombinase activity in the presence and absence of phosphatidylserine. PT nanodiscs strongly bound FVa and lactadherin (nM affinity) but poorly bound prothrombin and FX, suggesting that PT supports prothrombinase through recruitment of FVa. PT liposomes bearing tissue factor poorly generated thrombin in platelet poor plasma, indicating that PT poorly supports extrinsic tenase activity. On platelet activation, PT is externalized and partially metabolized. Last, PT was significantly higher in platelets and extracellular vesicle from patients with coronary artery disease than in healthy controls. In summary, PT is present in human blood, binds FVa and lactadherin, supports coagulation in vitro through FVa binding, and is elevated in atherosclerotic vascular disease. Our studies reveal a new phospholipid subclass, that contributes to the procoagulant membrane, and may support thrombosis in patients at elevated risk.

Immature platelets in COVID-19

Platelets play a critical role in immune response. Coronavirus disease 2019 (COVID-19) patients with a severe course often show pathological coagulation parameters including thrombocytopenia, and at the same time the proportion of immature platelets increases. In this study, the platelet count and the immature platelet fraction (IPF) of hospitalized patients with different oxygenation requirements was investigated daily over a course of 40 days. In addition, the platelet function of COVID-19 patients was analyzed. It was found that the number of platelets in patients with the most severe course (intubation and extracorporeal membrane oxygenation (ECMO)) was significantly lower (111.5 ∙ 10⁶ /mL) than in the other groups (mild (no intubation, no ECMO): 203.5 ∙ 10⁶ /mL, p < .0001, moderate (intubation, no ECMO): 208.0 ∙ 10⁶ /mL, p < .0001). IPF tended to be elevated (10.9%). Platelet function was reduced. Differentiation by outcome revealed that the deceased patients had a highly significant lower platelet count and higher IPF (97.3 ∙ 10⁶ /mL, p < .0001, 12.2%, p = .0003).

Platelet and mitochondrial RNA is decreased in plasma-derived extracellular vesicles in women with preeclampsia-an exploratory study

BACKGROUND

Circulating extracellular vesicles (EVs) are increased in preeclampsia (PE) and are associated with severity and progression. We examined in this exploratory cohort study if the mRNAs and long noncoding RNAs (lncRNAs) in plasma-derived EVs were dysregulated in PE compared to normal pregnancy and display different temporal patterns during gestation.

METHODS

We isolated EVs from plasma at weeks 22-24 and 36-38 in women with and without PE (n=7 in each group) and performed RNA-seq, focusing on mRNAs and lncRNAs. We validated highly expressed mitochondrial and platelet-derived RNAs discovered from central pathways in 60 women with/without PE. We examined further one of the regulated RNAs, noncoding mitochondrially encoded tRNA alanine (MT-TA), in leukocytes and plasma to investigate its biomarker potential and association with clinical markers of PE.

RESULTS

We found abundant levels of platelet-derived and mitochondrial RNAs in EVs. Expression of these RNAs were decreased and lncRNAs increased in EVs from PE compared to without PE. These findings were further validated by qPCR for mitochondrial RNAs MT-TA, MT-ND2, MT-CYB and platelet-derived RNAs PPBP, PF4, CLU in EVs. Decreased expression of mitochondrial tRNA MT-TA in leukocytes at 22-24 weeks was strongly associated with the subsequent development of PE.

CONCLUSIONS

Platelet-derived and mitochondrial RNA were highly expressed in plasma EVs and were decreased in EVs isolated from women with PE compared to without PE. LncRNAs were mostly increased in PE. The MT-TA in leukocytes may be a useful biomarker for prediction and/or early detection of PE.

Sources of variability in the human platelet transcriptome

Platelets are anucleated cells produced by megakaryocytes, from which they inherit all the components necessary to carry their functions. They circulate in blood vessels where they play essential roles in coagulation, wound repair or inflammation, and have been implicated in various pathological conditions such as thrombosis, viral infection or cancer progression. The importance of these cells has been established over a century ago, and effective anti-platelet medications with different mechanisms of action have since been developed. However, these therapies are not always effective and can incur adverse events, thus a better understanding of platelets molecular processes is needed to address these issues and improve our understanding of platelet functions. In recent years, an increasing number of studies have leveraged OMICs technologies to analyze their content and identify molecular signatures and mechanisms associated with platelet functions and platelet related disorders. In particular, the increased accessibility of microarrays and RNA sequencing opened the way for studies of the platelet transcriptome under a wide array of conditions. These studies revealed distinct expression profiles in diverse pathologies, which could lead to the discovery of novel biomarkers and therapeutic targets, and suggests a dynamic transcriptome that could influence platelet mechanisms. In this review, we highlight the different sources of transcript level variability in platelets while summarizing recent advances and discoveries from this emerging field.

Age-Dependent Surface Receptor Expression Patterns in Immature Versus Mature Platelets in Mouse Models of Regenerative Thrombocytopenia

Aging is a multifaceted process that unfolds at both the individual and cellular levels, resulting in changes in platelet count and platelet reactivity. These alterations are influenced by shifts in platelet production, as well as by various environmental factors that affect circulating platelets. Aging also triggers functional changes in platelets, including a reduction in RNA content and protein production capacity. Older individuals and RNA-rich immature platelets often exhibit hyperactivity, contributing significantly to pathologic conditions such as cardiovascular diseases, sepsis, and thrombosis. However, the impact of aging on surface receptor expression of circulating platelets, particularly whether these effects vary between immature and mature platelets, remains largely unexplored. Thus, we investigated the expression of certain surface and activation receptors on platelets from young and old mice as well as on immature and mature platelets from mouse models of regenerative thrombocytopenia by flow cytometry. Our findings indicate that aged mice show an upregulated expression of the platelet endothelial cell adhesion molecule-1 (CD31), tetraspanin-29 (CD9), and Toll-like receptor 2 (TLR2) compared to their younger counterparts. Interestingly, when comparing immature and mature platelets in both young and old mice, no differences were observed in mature platelets. However, immature platelets from young mice displayed higher surface expression compared to immature platelets from old mice. Additionally, in mouse models of regenerative thrombocytopenia, the majority of receptors were upregulated in immature platelets. These results suggest that distinct surface receptor expressions are increased on platelets from old mice and immature platelets, which may partially explain their heightened activity and contribute to an increased thrombotic risk.

Laboratory Markers of Platelet Production and Turnover

Platelets are formed from bone marrow megakaryocytes, circulate in blood for 7-10 days, and then are destroyed in the spleen and/or liver. Platelet production depends on the megakaryocyte population state in the bone marrow: number and size of the cells. The platelet turnover, i.e., the number of platelets passing through the bloodstream in a certain time, is determined by both the rate of their production and the rate of their destruction. The review considers laboratory markers, which are used to assess platelet production and turnover in the patients with hematologic and cardiovascular pathologies. These markers include some characteristics of platelets themselves: (i) content of reticulated ("young") forms in the blood detected by their staining with RNA dyes; (ii) indicators of the platelet size determined in hematology analyzers (mean volume, percentage of large forms) and in flow cytometers (light scattering level). Alterations of platelet production and turnover lead to the changes in blood plasma concentrations of such molecules as thrombopoietin (TPO, main mediator of megakaryocyte maturation and platelet formation in the bone marrow) and glycocalicin (soluble fragment of the membrane glycoprotein Ib detached from the surface of platelets during their destruction). Specific changes in the markers of platelet production and turnover have been observed in: (i) hypoproductive thrombocytopenias caused by suppression of megakaryocytes in the bone marrow; (ii) immune thrombocytopenias caused by accelerated clearance of the autoantibody-sensitized platelets; and (iii) thrombocytosis (both primary and reactive). The paper presents the data indicating that in patients with cardiovascular diseases an increased platelet turnover and changes in the corresponding markers (platelet size indexes and content of reticulated forms) are associated with the decreased efficacy of antiplatelet drugs and increased risk of thrombotic events, myocardial infarction, and unstable angina (acute coronary syndrome).

The role of platelets, neutrophils and endothelium in COVID-19 infection

INTRODUCTION

COVID-19 is associated to an increased risk of thrombosis, as a result of a complex process that involves the activation of vascular and circulating cells, the release of soluble inflammatory and thrombotic mediators and blood clotting activation.

AREAS COVERED

This article reviews the pathophysiological role of platelets, neutrophils and the endothelium, and of their interactions, in the thrombotic complications of COVID-19 patients, and the current and future therapeutic approaches targeting these cell types.

EXPERT OPINION

Virus-induced platelet, neutrophil and endothelial cell changes are crucial triggers of the thrombotic complications and of the adverse evolution of COVID-19. Both the direct interaction with the virus and the associated cytokine storm concur to trigger cell activation in a classical thromboinflammatory vicious circle. Although heparin has proven to be an effective prophylactic and therapeutic weapon for the prevention and treatment of COVID-19-associated thrombosis, it acts downstream of the cascade of events triggered by SARS-CoV-2. The identification of specific molecular targets interrupting the thromboinflammatory cascade upstream, and more specifically acting either on the interaction of SARS-CoV-2 with blood and vascular cells or on the specific signalling mechanisms associated with their COVID-19-associated activation, might theoretically offer greater protection with potentially lesser side effects.

The Analysis of the Human Megakaryocyte and Platelet Coding Transcriptome in Healthy and Diseased Subjects

Platelets are generated and released into the bloodstream from their precursor cells, megakaryocytes that reside in the bone marrow. Though platelets have no nucleus or DNA, they contain a full transcriptome that, during platelet formation, is transported from the megakaryocyte to the platelet. It has been described that transcripts in platelets can be translated into proteins that influence platelet response. The platelet transcriptome is highly dynamic and has been extensively studied using microarrays and, more recently, RNA sequencing (RNA-seq) in relation to diverse conditions (inflammation, obesity, cancer, pathogens and others). In this review, we focus on bulk and single-cell RNA-seq studies that have aimed to characterize the coding transcriptome of healthy megakaryocytes and platelets in humans. It has been noted that bulk RNA-seq has limitations when studying in vitro-generated megakaryocyte cultures that are highly heterogeneous, while single-cell RNA-seq has not yet been applied to platelets due to their very limited RNA content. Next, we illustrate how these methods can be applied in the field of inherited platelet disorders for gene discovery and for unraveling novel disease mechanisms using RNA from platelets and megakaryocytes and rare disease bioinformatics. Next, future perspectives are discussed on how this field of coding transcriptomics can be integrated with other next-generation technologies to decipher unexplained inherited platelet disorders in a multiomics approach.

Distinct Platelet RNA Signatures in Patients with Pulmonary Hypertension

RATIONALE

Pulmonary hypertension encompasses progressive disorders leading to right ventricular dysfunction and early death. Late detection is an important cause of poor clinical outcomes. However, biomarkers that accurately predict the presence of pulmonary hypertension are currently lacking.

OBJECTIVES

In this study we provide evidence that blood platelets contain a distinctive RNA profile that may be exploited for detection of pulmonary hypertension.

METHODS

Blood platelet RNA was isolated prospectively from 177 prevalent patients with different subtypes of pulmonary hypertension as well as 195 controls clinically not suspected of pulmonary hypertension. Sequencing libraries were created using SMARTer cDNA amplification, and sequenced on the Illumina HiSeq platform. RNA-sequencing reads were mapped to the human reference genome, and intron-spanning spliced RNA reads were selected. Differential spliced RNA panels were calculated by ANOVA-statistics. A particle swarm optimisation (PSO)-enhanced classification algorithm was built employing a development (n=213 samples) and independent validation series (n=159 samples).

RESULTS

We detected a total of 4014 different RNAs in blood platelets from pulmonary hypertension patients (n=177) and asymptomatic controls (n=195). GSEA gene ontology analysis revealed enriched RNA levels for genes related to RNA-processing, translation and mitochondrial function. A PSO-selected RNA panel of 408 distinctive differentially spliced RNAs mediated detection of pulmonary hypertension with 93% sensitivity, 62% specificity, 77% accuracy, 0.89 (95%CI 0.83-0.93) area under the curve and a negative predictive value of 91% in the independent validation series. Prediction score was independent of age, sex, smoking, pulmonary hypertension subtype, and the use of pulmonary hypertension-specific medication or anti-coagulants.

CONCLUSION

A platelet RNA-panel may accurately discriminate patients with pulmonary hypertension from asymptomatic controls. In the light of current diagnostic delays, this study is the starting point for further development and evaluation of a platelet RNA-based blood test, to ultimately improve early diagnosis and clinical outcomes in patients with pulmonary hypertension.

Role of Reticulated Platelets in Cardiovascular Disease

Human platelets differ considerably with regard to their size, RNA content and thrombogenicity. Reticulated platelets (RPs) are young, hyper-reactive platelets that are newly released from the bone marrow. They are larger and contain more RNA compared to older platelets. In comparison to more mature platelets, they exhibit a significantly higher thrombogenicity and are known to be elevated in patients with an increased platelet turnover such as, diabetics and after acute myocardial infarction. Several studies have shown that RPs correlate with an insufficient antiplatelet response to aspirin and specific P2Y₁₂ receptor inhibitors. In addition, RPs are promising novel biomarkers for the prediction of adverse cardiovascular events in cardiovascular disease. However, the reason for RPs intrinsic hyper-reactivity and their association with ischemic events is not completely understood and the biology of RPs is still under investigation. We here present a structured review of preclinical and clinical findings concerning the role of RPs in cardiovascular disease.

Emerging Technologies for Understanding Platelet Diversity

This review discusses our understanding of platelet diversity with implications for the roles of platelets in hemostasis and thrombosis and identifies advanced technologies set to provide new insights. We use the term diversity to capture intrasubject platelet variability that can be intrinsic or governed by the environment and lead to a heterogeneous response pattern of aggregation, clot promotion, and external communication. Using choice examples, we discuss how the use of advanced technologies can provide new insights into the underlying causes of platelet molecular, structural, and functional diversity. As sources of diversity, we discuss the proliferating megakaryocytes with different allele-specific expression patterns, the asymmetrical formation of proplatelets, changes in platelets induced by aging and priming, interplatelet heterogeneity in thrombus organization and stability, and platelet-dependent communications. We provide indications how current knowledge gaps can be addressed using promising technologies, such as next-generation sequencing, proteomic approaches, advanced imaging techniques, multicolor flow and mass cytometry, multifunctional microfluidics assays, and organ-on-a-chip platforms. We then argue how this technology base can aid in characterizing platelet populations and in identifying platelet biomarkers relevant for the treatment of cardiovascular disease.

Immune-Mediated Platelet Activation in COVID-19 and Vaccine-Induced Immune Thrombotic Thrombocytopenia

Both qualitative and quantitative platelet abnormalities are common in patients with coronavirus disease 2019 (COVID-19) and they correlate with clinical severity and mortality. Activated platelets contribute to the prothrombotic state in COVID-19 patients. Several groups have shown immune-mediated activation of platelets in critically ill COVID-19 patients. Vaccine-induced immune thrombotic thrombocytopenia is an autoimmune condition characterized by thrombocytopenia and life-threatening thrombotic events in the arterial and venous circulation. Although the initial trigger has yet to be determined, activation of platelets by immune complexes through Fc gamma RIIA results in platelet consumption and thrombosis. A better understanding of platelet activation in COVID-19 as well as in vaccine-induced thrombotic complications will have therapeutic implications. In this review, we focused on the role of immune-mediated platelet activation in thrombotic complications during COVID-19 infection and vaccine-induced immune thrombotic thrombocytopenia.

Impact of high platelet turnover on the platelet transcriptome: Results from platelet RNA-sequencing in patients with sepsis

Background

Sepsis is associated with high platelet turnover and elevated levels of immature platelets. Changes in the platelet transcriptome and the specific impact of immature platelets on the platelet transcriptome remain unclear. Thus, this study sought to address whether and how elevated levels of immature platelets affect the platelet transcriptome in patients with sepsis.

Methods

Blood samples were obtained from patients with sepsis requiring vasopressor therapy (n = 8) and from a control group of patients with stable coronary artery disease and otherwise similar demographic characteristics (n = 8). Immature platelet fraction (IPF) was determined on a Sysmex XE 2100 analyser and platelet function was tested by impedance aggregometry. RNA from leukocyte-depleted platelets was used for transcriptome analysis by Next Generation Sequencing integrating the use of unique molecular identifiers.

Results

IPF (median [interquartile range]) was significantly elevated in sepsis patients (6.4 [5.3–8.7] % vs. 3.6 [2.6–4.6] %, p = 0.005). Platelet function testing revealed no differences in adenosine diphosphate- or thrombin receptor activating peptide-induced platelet aggregation between control and sepsis patients. Putative circular RNA transcripts were decreased in platelets from septic patients. Leukocyte contamination defined by CD45 abundance levels in RNA-sequencing was absent in both groups. Principal component analysis of transcripts showed only partial overlap of clustering with IPF levels. RNA sequencing showed up-regulation of 524 and down-regulation of 118 genes in platelets from sepsis patients compared to controls. Upregulated genes were mostly related to catabolic processes and protein translation. Comparison to published platelet transcriptomes showed a large overlap of changes observed in sepsis and COVID-19 but not with reticulated platelets from healthy donors.

Conclusions

Patients with sepsis appear to have a less degraded platelet transcriptome as indicated by increased levels of immature platelets and decreased levels of putative circular RNA transcripts. The present data suggests that increased protein translation is a characteristic mechanism of systemic inflammation.

Hematopoietic responses to SARS-CoV-2 infection

Under physiological conditions, hematopoietic stem and progenitor cells (HSPCs) in the bone marrow niches are responsible for the highly regulated and interconnected hematopoiesis process. At the same time, they must recognize potential threats and respond promptly to protect the host. A wide spectrum of microbial agents/products and the consequences of infection-induced mediators (e.g. cytokines, chemokines, and growth factors) can have prominent impact on HSPCs. While COVID-19 starts as a respiratory tract infection, it is considered a systemic disease which profoundly alters the hematopoietic system. Lymphopenia, neutrophilia, thrombocytopenia, and stress erythropoiesis are the hallmark of SARS-CoV-2 infection. Moreover, thrombocytopenia and blood hypercoagulability are common among COVID-19 patients with severe disease. Notably, the invasion of erythroid precursors and progenitors by SARS-CoV-2 is a cardinal feature of COVID-19 disease which may in part explain the mechanism underlying hypoxia. These pieces of evidence support the notion of skewed steady-state hematopoiesis to stress hematopoiesis following SARS-CoV-2 infection. The functional consequences of these alterations depend on the magnitude of the effect, which launches a unique hematopoietic response that is associated with increased myeloid at the expense of decreased lymphoid cells. This article reviews some of the key pathways including the infectious and inflammatory processes that control hematopoiesis, followed by a comprehensive review that summarizes the latest evidence and discusses how SARS-CoV-2 infection impacts hematopoiesis.

Are platelet volume indices of clinical use in COVID-19? A systematic review

Background

The incidence of thrombotic complications is high in COVID-19 patients with severe disease. As key regulators of thrombus formation, platelets likely play a crucial role as mediators of severe acute respiratory syndrome coronavirus 2 associated pathogenesis. Studies have reported that parameters reflecting platelet size, known as platelet volume indices (PVI), are raised in patients with thrombosis and can predict poor outcomes. This systematic review evaluates the potential for PVI to be used as a predictor of COVID-19 morbidity and mortality.

Methods

English and Chinese databases were searched electronically to identify studies reporting data on mean platelet volume, platelet distribution width or platelet-large cell ratio in COVID-19 patients. Included articles underwent a quality rating and descriptive narrative analysis.

Results

Thirty-two studies were included in the systematic review. The results show a general trend for PVI to be raised in severe COVID-19 patients and non-survivors, with 14 studies reporting significant differences of baseline PVI between severe and mild disease. Nonetheless, longitudinal studies showed varying PVI trends over the course of the disease and evidence for PVI to be associated with disease progression was limited. The quality rating of 12 studies was poor, 16 were rated fair and four were good. Most studies were retrospective in design, used small study populations and did not consider confounding factors that influence platelet volume. Studies also contained technical flaws in PVI measurement, limiting the reliability of the results.

Conclusion

The evidence on the clinical usefulness of PVI is greatly limited by the lack of prospective evaluation, together with technical problems in measuring PVI. Carefully designed prospective studies are warranted.

Systematic review registration

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=304305, identifier CRD42022304305.

The α2-adrenergic receptor pathway modulating depression influences the risk of arterial thrombosis associated with BDNFVal66Met polymorphism

Depression is associated with thrombotic risk and arterial events, its proper management is strongly recommended in coronary artery disease (CAD) patients. We have previously shown that the Brain-Derived Neurotrophic Factor (BDNF)Val66Met polymorphism, related to depression, is associated with arterial thrombosis in mice, and with an increased risk of acute myocardial infarction in humans. Herein, expanding the previous findings on BDNFVal66Met polymorphism, we show that desipramine, a norepinephrine reuptake-inhibitor, rescues behavioral impairments, reduces the arterial thrombosis risk, abolishes pathological coagulation and platelet hyper-reactivity, normalizes leukocyte, platelet, and bone marrow megakaryocyte number and restores physiological norepinephrine levels in homozygous knock-in BDNF Val66Met (BDNF^Met/Met) mice. The in vitro data confirm the enhanced procoagulant activity and the alpha_2A-adrenergic receptor (α_2A-ADR) overexpression found in BDNF^Met/Met mice and we provide evidence that, in presence of Met variant, norepinephrine is crucial to up-regulate procoagulant activity and to enhance platelet generation. The α₂-ADR antagonist rauwolscine rescues the prothrombotic phenotype in BDNF^Met/Met mice and reduces procoagulant activity and platelet generation in cells transfected with BDNF^Met plasmid or exposed to pro-BDNF^Met peptide. Finally, we show that homozygous BDNF^Met/Met CAD patients have hyper-reactive platelets overexpressing abundant α_2A-ADR. The great proplatelet release from their megakaryocytes well reflects their higher circulating platelet number compared to BDNF^Val/Val patients. These data reveal an unprecedented described role of Met allele in the dysregulation of norepinephrine/α_2A-ADR pathway that may explain the predisposition to arterial thrombosis. Overall, the development of α_2A-ADR inhibitors might represent a pharmacological treatment for depression-associated thrombotic conditions in this specific subgroup of CAD patients.

Proteome and functional decline as platelets age in the circulation

BACKGROUND

Platelets circulate in the blood of healthy individuals for approximately 7-10 days regulated by finely balanced processes of production and destruction. As platelets are anucleate we reasoned that their protein composition would change as they age and that this change would be linked to alterations in structure and function.

OBJECTIVE

To isolate platelets of different ages from healthy individuals to test the hypothesis that changes in protein content cause alterations in platelet structure and function.

METHODS

Platelets were separated according to thiazole orange fluorescence intensity as a surrogate indicator of mRNA content and so a marker of platelet age and then subjected to proteomics, imaging, and functional assays to produce an in-depth analysis of platelet composition and function.

RESULTS

Total protein content was 45 ± 5% lower in old platelets compared to young platelets. Predictive proteomic pathway analysis identified associations with 28 biological processes, notably higher hemostasis in young platelets whilst apoptosis and senescence were higher in old platelets. Further studies confirmed platelet ageing was linked to a decrease in cytoskeletal protein and associated capability to spread and adhere, a reduction in mitochondria number, and lower calcium dynamics and granule secretion.

CONCLUSIONS

Our findings demonstrate changes in protein content are linked to alterations in function as platelets age. This work delineates physical and functional changes in platelets as they age and serves as a base to examine differences associated with altered mean age of platelet populations in conditions such as immune thrombocytopenia and diabetes.

Vitamin D and Platelets: A Menacing Duo in COVID-19 and Potential Relation to Bone Remodeling

Global data correlate severe vitamin D deficiency with COVID-19-associated coagulopathy, further suggesting the presence of a hypercoagulable state in severe COVID-19 patients, which could promote thrombosis in the lungs and in other organs. The feedback loop between COVID-19-associated coagulopathy and vitamin D also involves platelets (PLTs), since vitamin D deficiency stimulates PLT activation and aggregation and increases fibrinolysis and thrombosis. Vitamin D and PLTs share and play specific roles not only in coagulation and thrombosis but also during inflammation, endothelial dysfunction, and immune response. Additionally, another ‘fil rouge’ between vitamin D and PLTs is represented by their role in mineral metabolism and bone health, since vitamin D deficiency, low PLT count, and altered PLT-related parameters are linked to abnormal bone remodeling in certain pathological conditions, such as osteoporosis (OP). Hence, it is possible to speculate that severe COVID-19 patients are characterized by the presence of several predisposing factors to bone fragility and OP that may be monitored to avoid potential complications. Here, we hypothesize different pervasive actions of vitamin D and PLT association in COVID-19, also allowing for potential preliminary information on bone health status during COVID-19 infection.

Immature platelets in patients with Covid-19: association with disease severity

Coronavirus disease 2019 (Covid-19) is associated with a high incidence of venous and arterial thromboembolic events. Currently, there are no clinical or laboratory markers that predict thrombotic risk. Circulating immature platelets are hyper-reactive platelets, which are associated with arterial thrombotic events. The aim of this study was to assess whether the proportion of circulating immature platelets is associated with disease severity in Covid-19 patients. Patients admitted with Covid-19 disease were prospectively assessed. Immature platelet count (IPC) and immature platelet fraction (IPF) were measured at admission and at additional time points during the hospital course using the Sysmex XN-3000 auto-analyzer. A total of 136 consecutive patients with Covid-19 were recruited [mean age 60 ± 19 years, 49% woman, 56 (41%) had mild-moderate disease and 80 (59%) had severe disease at presentation]. The median IPF% was higher in patients with severe compared to mild-moderate disease [5.8 (3.9-8.7) vs. 4.2 (2.73-6.45), respectively, p = 0.01]. The maximal IPC value was also higher in patients with severe disease [15 (10.03-21.56), vs 10.9 (IQR 6.79-15.62), respectively, p = 0.001]. Increased IPC was associated with increased length of hospital stay. Patients with severe Covid-19 have higher levels of IPF than patients with mild-moderate disease. IPF may serve as a prognostic marker for disease severity in Covid-19 patients.

Platelets in the perspective of COVID-19; pathophysiology of thrombocytopenia and its implication as prognostic and therapeutic opportunity

Despite endorsed and exponential research to improve diagnostic and therapeutic strategies, efforts have not yet converted into a better prospect for patients infected with the novel coronavirus (2019nCoV), and still, the name of SARS-CoV-2 is coupled with numerous unanswered questions. One of these questions is concerning how this respiratory virus reduces the number of platelets (PLTs)? The results of laboratory examinations showed that about a quarter of COVID-19 cases experience thrombocytopenia, and more remarkably, about half of these patients succumb to the infection due to coagulopathy. These findings have positioned PLTs as a pillar in the management as well as stratifying COVID-19 patients; however, not all the physicians came into a consensus about the prognostic value of these cells. The current review aims to unravel the contributory role of PLTs s in COVID-19; and also to summarize the original data obtained from international research laboratories on the association between COVID-19 and PLT production, activation, and clearance. In addition, we provide a special focus on the prognostic value of PLTs and their related parameters in COVID-19. Questions on how SARS-CoV-2 induces thrombocytopenia are also responded to. The last section provides a general overview of the most recent PLT- or thrombocytopenia-related therapeutic approaches. In conclusion, since SARS-CoV-2 reduces the number of PLTs by eliciting different mechanisms, treatment of thrombocytopenia in COVID-19 patients is not as simple as it appears and serious cautions should be considered to deal with the problem through scrutiny awareness of the causal mechanisms.

Platelet reticulated forms, size indexes, and functional activity. Interactions in healthy volunteers

Reticulated platelets (RP) are young, functionally active platelet forms which are detected by RNA staining. Their content in the circulation reflects the intensity of bone marrow thrombocytopoesis. The aim of this study was to assess in healthy volunteers the relationship between RP percentage and platelet size and activity. RP were quantitated by thiazole orange staining using flow cytometry. Platelet size indexes included mean platelet volume (MPV), platelet large cell ratio (P-LCR) measured in a Coulter type hematological analyzer and forward scattering (FSC) measured in a flow cytometer. Platelet functional activity was evaluated by expression of activated glycoprotein (GP) IIb-IIIa (PAC-1 antibody binding) and P-selectin with the use of flow cytometry. Platelets were activated by thrombin receptor activating peptide (TRAP) (10 and 1 µM) and ADP (20 and 2.5 µM). The percentage of RP in healthy volunteers varied from 2.9% to 23.8% (mean ± SD ‒ 11.7 ± 4.7%, n = 99) and correlated with all platelet size indexes: MPV, P-LCR and FCS (r from 0.452 to 0.529, p < .001, n = 87-99). On average, RP were distributed at a ratio of 9:1 between 50% subpopulations of large and small platelets according to their FSC index. Expression of GP IIb-IIIa activated form correlated with RP percentage and platelet size indexes when platelets were activated by TRAP and ADP at both applied concentrations (r from 0.309 to 0.560, p from 0.014 to < 0.001, n = 50-62). P-selectin expression correlated with RP percentage and platelet size indexes when platelets were activated by 10 µM TRAP inducing maximum expression of this activation marker (r from 0.332 to 0.556, p from 0.008 to < 0.001, n = 65), but not by weaker agonists: 1 µM TRAP, 20 and 2.5 µM ADP (r < 0.3, n = 54-66). Thus, high RP content in healthy volunteers is associated with increased platelet size and activity in the whole platelet population.

The multifaceted contribution of platelets in the emergence and aftermath of acute cardiovascular events

Atherosclerosis is an underlying cause of a broad array of cardiovascular diseases characterized by plaques, arterial wall thickening initiated by hyperlipidemia, pro-inflammatory signals, endothelial dysfunction and the influx of inflammatory cells. By still incompletely characterized mechanisms, these plaques can destabilize or erode, leading to thrombosis and blood vessel occlusion and becomes clinically manifest as angina pectoris, myocardial infarction (MI) or stroke. Among the several blood cell types that are involved in the development of atherosclerosis, the role of platelets during the thrombotic occlusion of ruptured or eroded plaques is well established and clinically exploited as evident by the extensive use of platelet inhibitors. However, there is increasing evidence that platelets are also involved in the earlier stages of atheroma development by exhibiting pro-inflammatory activities. The scope of this review is to describe the role of platelets in the initiation and propagation stages of atherosclerosis and beyond; in atherothrombotic complications.

Reticulated Platelets in Medicine: Current Evidence and Further Perspectives

Reticulated platelets (RPs) are young thrombocytes, newly released from the bone marrow. The identification and quantification of these cells remained difficult for decades due to a lack of standardized preanalytical and analytical methods. With the introduction of automated hematology analyzers in clinical routine, the determination of RPs, either as a total count or as a fraction, became more reliable, faster and more affordable. Currently, RPs are the focus of research in multiple clinical settings. In cardiovascular medicine, recent studies have focused on the relationship between RPs, coronary artery disease (CAD) and clinical outcomes, as well as the impact of RPs on the effects of antiplatelet therapy. Cohort studies showed increased levels of RPs in patients with acute coronary syndrome (ACS) or cardioembolic stroke. In patients with ACS, increased levels of RPs were also associated with an increased incidence of major ischemic cardiovascular events during follow-up. Further studies showed an association of levels of RPs with the antiplatelet response to less-potent P2Y12 inhibitors. In patients with paroxysmal atrial fibrillation undergoing pulmonary vein isolation, levels of RPs differed significantly depending on the achieved rhythm (sinus rhythm vs. recurrent atrial fibrillation). Levels of RPs appear to also be predictive for bleeding events in patients with various hematological diagnoses. Although no causal relationship has so far been proven, RP values have been associated with a large number of pathologies and clinical scenarios. This review summarizes the current evidence with regard to RPs and their potential diagnostic and prognostic value for noncardiovascular patients and for cardiovascular patients in particular. It describes further perspectives on how the testing of these cells might improve the treatment of cardiovascular patients.

The Impact of COVID-19 Disease on Platelets and Coagulation

Coronavirus disease 2019 (COVID-19) causes a spectrum of disease; some patients develop a severe proinflammatory state which can be associated with a unique coagulopathy and procoagulant endothelial phenotype. Initially, COVID-19 infection produces a prominent elevation of fibrinogen and D-dimer/fibrin(ogen) degradation products. This is associated with systemic hypercoagulability and frequent venous thromboembolic events. The degree of D-dimer elevation positively correlates with mortality in COVID-19 patients. COVID-19 also leads to arterial thrombotic events (including strokes and ischemic limbs) as well as microvascular thrombotic disorders (as frequently documented at autopsy in the pulmonary vascular beds). COVID-19 patients often have mild thrombocytopenia and appear to have increased platelet consumption, together with a corresponding increase in platelet production. Disseminated intravascular coagulopathy (DIC) and severe bleeding events are uncommon in COVID-19 patients. Here, we review the current state of knowledge of COVID-19 and hemostasis.